Character generating device

ABSTRACT

In a character generating device in which character signals are generated by scanning a character plate having character slits therein, with an electron beam, a signal developed across a load connected with an electrode provided in front of the character plate with respect to the travel of the beam by collecting with said electrode the secondary electrons emitted from a portion of the surface of said character plate other than said character slits when said beam hits against that portion and a signal developed across a load connected with a collector electrode provided in rear of the character plate with respect to the travel of the electron beam by collecting with the character plate the secondary electrons emitted from the collector electrode when the beam passes through the character slits of the character plate and impinges on the collector electrode, have the same polarity and both the signals are added through respective coupling capacitors to produce a composite signal. The composite signal is opposite in polarity to the signal developed across the load connected with the character plate. Accordingly, if the composite signal and the signal across the load connected with the character plate are differentially combined and thereafter applied to an external circuit, then an output representing any character signal can be increased in magnitude. Consequently, since a high level output signal is obtained without increasing the electron beam current according to the proposed device, the resolution or character generating capacity is improved. If the resolution need not be so high, the accelerating voltage can be accordingly reduced.

United States Patent Kaji et al.

[ Aug. 21, 1973 [73] Assignee: Matsushita Electronics Corporation,

Osaka, Japan [22] Filed: Dec. 28, 1971 [21] Appl. No.: 213,126

[30] Foreign Application Priority Data Dec. 29, 1970 Japan 45/124856 [52] US. Cl 315/11, 315/12, 315/22, 315/30 [51] Int. Cl. 1101] 31/48 [58] Field of Search 315/11, 12, 18,22, 315/26. 30

[56] References Cited UNITED STATES PATENTS 3,054,927 9/1962 Phillips et al. 315/12 3,239,766 3/1966 Manley 315/12 X 3,551,731 12/1967 Harpster.... 315/12 3,336,498 8/1967 Castanera 315/18 3,439,215 4/1969 Brenner, Jr. 315/12 3,439,216 4/1969 Brenner, Jr. et al 3'15/12 3,577,171 5/1971 Turner 315/11 X Primary Examiner-Carl D. Quarforth Assistant ExaminerP. A. Nelson Attorney- S. Delvalle Goldsmith, Lester Horwitz et al.

[5 7] ABSTRACT In a character generating device in which character signals are generated by scanning a character plate having character slits therein, with an electron beam, a signal developed across a load connected with an electrode provided in front of the character plate with respect to the travel of the beam by collecting with said electrode the secondary electrons emitted from a portion of the surface of said character plate other than said character slits when said beam hits against that portion and a signal developed across a load connected with a collector electrode provided in rear of the character plate with respect to the travel of the electron beam by collecting with the character plate the secondary electrons emitted from the collector electrode when the beam passes through the character slits of the character plate and impinges on the collector electrode, have the same polarity and both the signals are added through respective coupling capacitors to produce a composite signal. The composite signal is opposite in polarity to the signal developed across the load connected with the character plate. Accordingly, if the composite signal and the signal across the load connected with the character plate are differentially combined and thereafter applied to an external circuit, then an output representing any character signal can be increased in magnitude. Consequently, since a high level output signal is obtained without increasing the electron beam current according to the proposed device, the resolution or character generating capacity is improved. 1f the resolution need notbe so high, the accelerating voltage can be accordingly reduced.

4 Claims, 7 Drawing Figures sc T Vs o Patented Aug. 21, 1973 4 Sheets-Sheet 1 PR/Of? ART FIG: 2 P/P/O ART Patented Aug. 21, 1973 4 Sheets-Sheet 2 F/G 3 PR/Of? ART Fla 4 PfP/Of? ART Patented Aug. 21, 1973 3,754,158

4 Sheets-Sheet 4 1 CHARACTER GENERATING DEVICE The present invention relates to a character generating device which generates character signals by scanning a target as a source of character information with an electron beam. In a conventional character generating device, a plate having cut-through slots corresponding to desired characters therein is placed in the path of a scanning electron beam, and if the beam scans a given slot corresponding to a certain character, then a signal representative of the character is derived. By applying the signal to the intensity modulating circuit of a picture tube which is synchronously connected with the scanning system, the character is visualized on the picture tube.

The present invention will now be described in detail in comparison with a conventional device and for better understanding the invention reference should now be made also to the accompaying drawings in which:

FIG. 1 is a schematic diagram of a conventional character generating device and its associated circuit;

FIG. 2 shows the wave form of the output signal from the device shown in FIG. 1;

FIG. 3 shows graphically the voltage-current characteristic of the device shown in FIG. 1;

FIG. 4 is a schematic diagram of another conventional character generating device and its associated circuit;

FIG. 5 is a schematic diagram of a character generating tube embodying the present invention and its associated circuit;

FIG. 6 shows the wave forms of output signals of the tube as shown in FIG. 5, appearing several places at points A, B, C and D of the circuit in FIG. 5; and

FIG 7 is a circuit diagram of another embodiment of the present invention.

In FIG. 1, which shows a conventional character generating device, a cathode 1, an anode 2, deflecting electrodes 3, accelerating electrodes 4, a character plate 5 and a collector electrode 6 are hermetically contained in an envelope 7, and a coupling capacitor 8 and an amplifier 9 are connected in series with the collector electrode 6 to which a resistor R and a dc source V, are connected in series as a load circuit. Inspecting FIG. 1, one can see that a signal current i, flows through the resistor R only when the electron beam passes through a perforated slit corresponding to each character formed in the character plate 5 and impinges upon the collector electrode 6 Therefore, the wave form of the signal current i is that which is graphically shown in FIG. 2. The coordinate system in FIG. 2 has, for example, time scale on the abscissa and current scale on the ordinate. A signal voltage v, i,R,, will appear at the point A in FIG. I when the electron-beam scans any character slit of the character plate 5. When the beam passing through the character slit impinges on the collector electrode 6, secondary electrons will be emitted from on the surface of the electrode 6. If in this case the collector electrode 6 is maintained at a sufficiently high positive potential the secondary electrons will be recollected by the electrode 6. Accordingly, the signal current i, is equal to a current i, due to the electron beam so that the output signal voltage v, becomes equivalent to the product of i and R i.e. v, i -R On the other hand, if the voltage V applied to the collector electrode 6 is reduced to a certain level, the secondary electrons emitted due to the impingement of the electron beam on the collector electrode 6 develop a tendency to be absorbed in the accelerating electrodes 4 so that the signal current i, will considerably be changed. FIG. 3 shows the condition of such a change in the signal current i with the voltage V varying from a high positive level to a level equal to the potential V,, at the cathode 1. It is noted from FIG. 3 that if the voltage V applied to the collector electrode 6 is slightly more positive than the potential V,, at the cathode 1 then the electron beam passes through the character slits of the character plate 5 and impinges on the collector electrode 6. If the kinetic energy of electrons of the beam is small, the number of secondary electrons emitted from the collector electrode 6 is small so that the signal current i, assumes a positive value. With increasing voltage V, the energy of the electron beam also increases with the result that secondary emission accordingly increases. And if the potential at the collector electrode 6 is lower than that at the accelerating electrode 4 and the character plate 5 which are maintained at the same potential, all or part of emitted secondary electrons will be absorbed by the accelerating electrode 4 and/or the character plate 5. As a result, the signal current i, flowing into the collector electrode 6 will increase. If the potential at the collector electrode 6 is further elevated up to a level equal to or higher than that at the accelerating electrode 4, emitted secondary electrons are attracted and recollected by the collector electrode 6 due to the potential relationship between the accelerating electrode 4 and the collector electrode 6.

In FIG. 3, curves a, b and c represent the characteristics for the collector electrode made of stainless steel, Nesa film which is highly emissive of secondary electrons and carbon which has a low rate of secondary electron emission, respectively. From the curves a and b in FIG. 3, it is noted that the signal current i, and therefore the corresponding signal voltage v, which may be made negative as well as positive by controlling the voltage V applied to the collector electrode, can be derived. Therefore, it is possible to invert the contrast of a character image to be displayed on the picture tube. The curve d is the ideal characteristic for the collector electrode made of an ideal material which produces no secondary electron under condition in question. Actually, the characteristic can not be attained.

In order to obtain a stable output with the device described above notwithstanding the secondary emission from the collector electrode 6, the voltage V, applied to the electrode 6 had to be increased.

On the other hand, an attempt has been made to isolate the character plate 5 from the accelerating electrode 4 so as to have a larger output signal and to derive a signal from also the character plate. For such an example, reference should be made to the U. S. Pat. No. 3336498 specification, which discloses a device as shown in FIG. 4 of the attached drawing of the present specification. In this prior art device, in which the same reference characters and numerals indicate like parts or circuit elements as in the previously described device shown in FIG. 1, the collector electrode 6 and the character plate 5 are insulated from each other and energized by separate dc voltage sources V and V,. Now, if the voltages are so chosen that V,, V, 0, the currents drawn from the collector electrode 6 and the character plate 5 are free from the effect of secondary emission but are determined solely by the electron beam from the electron gun so that the signal voltage v i R appearing at the point A in FIG. 4 is equal in magnitude but opposite in polarity to the signal v i R at B in FIG. 4. If both the signal v and v,, are passed through coupling capacitors 8 and 8' to a differential amplifier 10 to be amplified therein, then the resultant output is twice as large as that obtained in case where the signal voltage is delivered from the collector electrode 6 alone.

According to the present invention, not only the primary electrons from the electron gun but also the secondary electrons due to the impingement of the beam on the collector electrode are utilized to obtain a larger output than those obtained by the above described prior art devic s. With increased output the current carried by the beam could be decreased. Therefore the image resolution is improved. Moreover, if the resolution need not be so high, the accelerating voltage can be accordingly lowered.

Next, the present invention will be described by way of examples its preferred embodiments, taken in conjunction with the accompanying drawings in which the same reference numerals and characters indicate like parts and circuit elements throughout.

FIG. 5 shows a character generating device embodying the present invention, in which in addition to the character plate 5 and the collector electrode 6, a secondary electron collector electrode 11 having a shape of a short hollow cylinder is provided in front of the character plate 5 as viewed from the electron gun, the collector electrode 6, the character plate 5 and the secondary electron collector electrode 11 having respectively dc voltages V V, and V, applied thereto through corresponding load resistors R R and R,, and the voltages being set such that V V, V, 0.

Now, when the electron beam hits against the nonperforated portion of the character plate, secondary electrons are emitted from that portion. Almost all the electrons are collected by the secondary electron collector electrode 1 1 since V, V As a result, the current from the source voltage V,, flows due to the secondary electrons, from the secondary electron collector electrode 11 to the character plate 5 so that the potential at the point C in FIG. 5 falls down while the potential at B rises up. On the other hand, when the electron beam passes through the character slits of the character plate 5, no current will flow between the secondary electron collector electrode 11 and the character plate 5 so that the potential at the point C is restored to the values V Under this condition the beam impinges on the collector electrode 6 and the resultant secondary electrons from the electrode 6 are absorbed in the character plate 5 to give rise to a current between the electrode 6 and the plate 5. Consequently, the potential at B falls while the potential at A rises. If the beam again impinges on a non-perforated portion of the character plate surface, the potential at A resumes the value V and those at B and C experience such changes as previously described. FIG. 6 shows the changes in the potentials at the points A, B and C with time of scanning. Provided that the character plate 5 and the collector electrode 6 are made of the same material and that all the secondary electrons emitted from the character plate 5 and the collector electrode 6 are absorbed by the secondary electron collector electrode 11 and the character plate 5 respectively, then the amounts of the corresponding changes in the potentials at A, C and B will be at values v,, v, and 2v,, respectively. Also the amount of change in the potential at A and that at C are added up to produce an amount 2v, of potential change at the point D. The potential at B is opposite in polarity to that at D so that if both are amplified together through the differential amplifier 10 an output signal corresponding to an input level 4v, will be delivered. Therefore, it is noted that according to the present invention, an FIG. signal can be obtained which is four times as large in magnitude as the output signal obtained by the prior art device as shown in FIG. 1 and twice as large as the output signal obtainable with the well known device as shown in FIG. 3. The value v, of the amount of the change in the potential at A or C depends on the current due to the secondary electrons emitted from the character plate 5 and the collector electrode 6. Consequently, by using a material having a high secondary-emission ratio for the character plate 5 and the collector electrode 6, the value of v can be made larger.

FIG. 7 shows a modification of the embodiment of the present invention shown in FIG. 5, in which the accelerating electrode 4 especially provided in the vicinity of the character electrode serves as the secondary electron collector electrode. In this case the dc voltages V and V, should preferably be set such that V, V, 0.

In conclusion the present invention provides a character generating device for generating a desired character signal by means of scanning with an electron beam, comprising an electron gun including a cathode, deflecting electrodes for the electron beam, a character plate having slits in the form of characters and a collector electrode, wherein a signal output due to the secondary electrons emitted from a portion of the character plate surface other than the character slits when the beam hits against that portion and a signal output due to the secondary electrons emitted from the collector electrode when the beam passes through the character slits of the plate and impinges on the collector electrode are respectively derived from an electrode means provided in the vicinity of the character plate and the collector electrode. According to the present invention, therefore, not only an output signal having a high level can be produced but also the character generating capacity is very much improved.

What we claim is:

1. A character signal generating device comprising a cylindrical envelope having an electron gun including a thermionic emission cathode positioned at one side of said envelope, means for deflecting electron beam from said cathode in response to external signal, character electrode positioned on the other side of said envelope and having its surface of a secondary emissive ratio higher than unity opposite to said gun and having perforations shaped in desired character, means positioned,between said gun and said character electrode for capturing secondary electrons, and collector electrode positioned at the back of said character electrode distant therefrom for collecting primary electrons passed through the perforations and having its surface of a secondary emission ratio higher than unity for emitting secondary electrons toward said character electrode by the incidence of said primary electrons; wherein said device further comprising first, second and third voltage supplies connected to said secondary electron capturing means, said character electrode and said collector electrode respectively through respective load resistors such that said first voltage is set more positive than said second voltage set more positive than said third voltage; and a differential amplifier having its first input terminal connected through a capacitor to said character electrode and its second input terminal connected to said collector electrode and said secondary capturing means through respective capacitors; whereby the character representative signals are concurrently applied from said load resistors to the first and second input terminals of said amplifier having therebetween a difference of summation of their absolute magnitude to increase a corresponding output of said amplifier.

trons.

l l i l t 

1. A character signal generating device comprising a cylindrical envelope having an electron gun including a thermionic emission cathode positioned at one side of said envelope, means for deflecting electron beam from said cathode in response to external signal, character electrode positioned on the other side of said envelope and having its surface of a secondary emissive ratio higher than unity opposite to said gun and having perforations shaped in desired character, means positioned between said gun and said character electrode for capturing secondary electrons, and collector electrode positioned at the back of said character electrode distant therefrom for collecting primary electrons passed through the perforations and having its surface of a secondary emission ratio higher than unity for emitting secondary electrons toward said character electrode by the incidence of said primary electrons; wherein said device further comprising first, second and third voltage supplies connected to said secondary electron capturing means, said character electrode and said collector electrode respectively through respective load resistors such that said first voltage is set more positive than said second voltage set more positive than said third voltage; and a differential amplifier having its first input terminal connected through a capacitor to said character electrode and its second input terminal connected to said collector electrode and said secondary capturing means through respective capacitors; whereby the character representative signals are concurrently applied from said load resistors to the first and second input terminals of said amplifier having therebetween a difference of summation of their absolute magnitude to increase a corresponding output of said amplifier.
 2. A device according to claim 1, wherein said secondary electron capturing means is formed in an independent cylindrical electrode.
 3. A device according to claim 1, wherein said envelope has an electron accelerating electrode positioned between said gun and said secondary electron capturing means which is formed in an independent cylindrical electrode.
 4. A device according to claim 1, wherein said secondary electron capturing means is formed in a cylindrical electrode also adapted for acceleration of electrons. 